The present invention relates to the field of plant breeding. More particularly, the present invention describes a lettuce plant type having the general shape of a romaine lettuce and having other qualities similar to iceberg lettuce. This new iceberg lettuce plant type has an elliptical plant shape, and spatulate shaped leaves generally recognizable in stature and appearance similar to romaine lettuce, instead of the normal spherical shaped plants with obovate shaped leaves of the iceberg. Other qualities of these plants, e.g., head weight, interior and exterior color, texture, density, and taste, are similar to iceberg lettuce.
Lettuce, Lactuca sativa L. is a commercially important fresh leaf crop belonging to the Cichoreae tribe of the aster family (Asteraceae) which includes such other important crops as sunflowers and artichokes. Lettuce is widely grown throughout the temperate and subtropical regions of the world and is used predominantly as a fresh green in the human diet.
There are six morphological types of lettuce: iceberg (crisphead), romaine (cos), butterhead, leaf, stem and Latin. Ryder, E. J., 1979, Leafy Salad Vegetables, AVI Publishing Company. These basic lettuce types frequently form the basis for grouping lettuces as is commonly seen in supermarkets, grocery and produce stores.
The crisphead type is the most common in the United States, while butterheads and romaines are the popular types in northern and southern Europe. Id. In the United States, California is the leading producer of lettuce. California produces crisphead, leaf and romaine lettuce. In 1995, California""s cash receipts for crisphead lettuce amounted to about $987 million dollars. 1996 California Agricultural Resource Directory. Furthermore, also in 1995, California exported about $154 million dollars of crisphead lettuce to other countries such as Japan, Canada, the European Union and Korea. Id.
Terms used herein to describe plants are explained in xe2x80x9cGuidelines for the Conduct of Tests for Distinctness, Homogeneity and Stabilityxe2x80x9d UPOV Ref. No. TG/13/7, Oct. 16, 1993, which is hereby incorporated by reference.
The iceberg group lettuces are characterized by their relatively large, spherical, dense heads averaging 20.0 cm in diameter and 1000 g in weight, which are borne on a set of frame leaves that form the base of the plant. The heads are composed of leaves that are spirally arranged on a stem with greatly foreshortened internodes, are tightly clasping upon one another forming a sphere of broadly shape (obovate) leaves, where the length and width of each leaf is nearly identical, having length to width ratio (L/W ratio) of 1.0 approximates 20.0 cm in length to 20.0 in width. The Length to width ratio for commercial iceberg lettuce varieties ranges from about 0.5 to 1.0. Outer leaves range in color intensity from dark green (Royal Horticultural Society Color Chart 146A) to green (RHS 146B) with inner leaves ranging from very pale green (RHS 145C) to white or blanched (RHS 145D). Iceberg lettuces have a closed head formation. Iceberg leaves have a high content of waterxe2x80x94hence the name, xe2x80x9ccrisphead.xe2x80x9d More information regarding the general characteristics of iceberg lettuce may be found in Ryder, E. J., Leafy Salad Vegetables, AVI Publishing Company, which is hereby incorporated by reference.
The romaine group of lettuces are characterized by large, cylindrical, semi-firm heads averaging 30.0 cm in diameter and 800 g in weight, which are borne on a set of frame leaves that form the base of the plant. The heads are composed of leaves that are spirally arranged on a stem with greatly foreshortened internodes, are loosely clasping upon one another forming a roll of elongated, spatula-shaped (spatulate) leaves, where the length is normally 50% longer than the width, having a range of length to width ratios of 1.2 to 2.5, where 1.5 is most common. Romaine lettuces generally have a semi-open head formation. The name xe2x80x9cromainexe2x80x9d comes from the French for xe2x80x9cRomanxe2x80x9d. Outer leaves range in color intensity from dark green (RHS 146A) to mid-green (RHS 146B) to light green (RHS 146C) with inner leaves ranging from green (RHS 146B) to light green (RHS 146D). More information regarding the general characteristics of romaine lettuce may be found in Ryder, E. J., Leafy Salad Vegetables, AVI Publishing Company.
A major disadvantage of existing iceberg varieties lies in their shape. While they are widely used in salads and sandwiches for their unique taste and crunchy texture, the iceberg""s round shape makes it difficult to process, resulting in inefficient handling and waste. For example, the iceberg lettuce head cannot be used when separating individual leaves to allow for cleaning and the production of individual lettuce leaves. The deeply cup-shaped leaves make thorough cleaning difficult and this same shape prevents their use as individual leaves in sandwiches because of their inability to lie flat and thus be are broken and damaged. In contrast, such disadvantages are not present in the shape of romaine lettuces.
Another disadvantage of existing iceberg varieties lies in their growing pattern. Because they grow low to the ground, harvesting must be done by hand. This means high harvest costs due to hand labor, which can also cause potential damage to the head when the lettuce stem is not cut at the correct level. Improperly harvested heads cannot be salvaged and must be discarded. In contrast, romaine lettuces grow erect and their leaves are oriented nearly vertically, thereby allowing for better access to the stem. These erect plants (such as a romaine) make the prospect of machine harvesting possible. Machine harvesting results in significant time and money savings.
Another disadvantage of existing iceberg varieties lies in their susceptibility to certain debilitating diseases. All plants rely on evaporation of moisture from the surface of their leaves to draw vital water and nutrients up into the remote areas were growth occurs. Due to the round head shape for crisphead lettuce, inner leaves are not exposed to light and air, thereby inhibiting evaporation of moisture from its inner leaves and the translocation of critically needed to move water, nutrients, and defensive agents to all interior areas of the head. Under these circumstances, the lack of calcium and other minerals in the process of leaf formation is a common problem in iceberg lettuce, causing brown and back spots (tipburn) to occur leading to the likelihood of subsequent infection by secondary pathogens such as bacteria and fungi, that result in the rapid decay and spoilage of the head from the inside out. In contrast, such diseases are much rarer in romaine lettuces due to the erect, semi-open shape of the head.
Other problems with existing cultivars adapted to western conditions include a lack of resistance to corky root rot and lettuce mosaic virus. Corky root rot is believed to be caused by a pathogenic soil bacterium of the genus Rhizomonas. One species of Rhizomonas that is commonly found to cause corky root rot is R. suberifaciens. Corky root rot accounts for significant lettuce crop loss in the western United States, particularly in the valleys of the central coast of California, i.e., the Salinas, Santa Maria, and Lompoc valleys.
Lettuce mosaic virus, on the other hand, is commonly found throughout the world, and occurs in all lettuce production areas of the United States. Vectored by the Green Peach aphid (Myzus persicae), outbreaks of lettuce mosaic virus can devastate an entire field within a short period of time.
Corky root rot symptoms include yellow bands on tap and lateral roots of lettuce seedlings. Guide to Leafy Vegetable Production in the Far West, Ron Smith, ed., California-Arizona Farm Press (1997). Yellow areas gradually expand and develop a green-brown color with cracks and rough areas on the root surface. The entire taproot may become brown, severely cracked and may cease to function. Feeder root systems are reduced and damaged. Roots become very brittle and break off easily. When the root is severely discolored, aboveground symptoms show up as wilting during warm temperatures, stunting and general poor, uneven growth. Loss of the root system results in stunted plants that are chlorotic and too small to harvest.
Lettuce mosaic virus symptoms first appear as vein clearing in the newly developed leaves followed by mottling (a mosaic appearance) and recurving of the leaves as they mature. Ryder, E. J., Leafy Salad Vegetables, AVI Publishing Company. The leaf margins increase in undulation and necrotic spots may also appear. Infected plants are generally much smaller than healthy plants, rendering the plants unsuitable for harvest.
The present invention relates to a new type of lettuce which combines a number of advantageous characteristics of romaine, e.g., a flat leaf shape, stature and semi-open headedness plus iceberg lettuce characteristics, e.g., large, spherical, firm heads with inner blanched leaves and a crunchy, bland texture.
The present invention relates to an iceberg lettuce plant having a first outer leaf having a length to width ratio of between about 1.2 and about 2.7. The present invention further relates to iceberg lettuce plants having spatulate leaf shape or an elliptical stature or a semi-open head formation. This invention is also directed to methods of producing an iceberg lettuce with a first outer leaf having a length to width ratio of between about 1.2 and about 2.7.
Definitionsxe2x80x94In the description and tables which follow, a number of terms are used. In order to provide a clear and consistent understanding of the specification and claims, including the scope to be given such terms, the following definitions are provided:
First outer leafxe2x80x94As described herein, xe2x80x9cfirst outer leafxe2x80x9d means the first leaf located on the outer surface of the lettuce head.
Leaf Length to Width Ratio (L/W Ratio)xe2x80x94As used herein, the length to width ratio is calculated by dividing the length of the first outer leaf by the width of the first outer leaf measured at the widest point.
Outer leafxe2x80x94As used herein, the term xe2x80x9couter leafxe2x80x9d refers to the 10 outer most leaves on a head of lettuce.
Inner leafxe2x80x94As used herein, the term xe2x80x9cinner leafxe2x80x9d refers to the 10 leaves closest to the core of a head of lettuce.
This invention provides a new type of iceberg lettuce having a plant and leaf shape and stature similar to romaine lettuce. At maturity, the heads of the plants of the present invention generally measure more than 20.0 cm in length and is similar to a typical romaine plant height, whereas normal iceberg heads rarely exceed 20.0 cm.
A common problem in the creation of new varieties is the very low occurrence of the desired genetic phenotype in a large genetic population. With more than 10,000 genes known to exist in plants, it is often highly improbable, if not impossible to converge on all the desired genetic traits in one individual. Prior to this work, it was not known whether the modified shape and stature of romaine could be incorporated into a commercially viable iceberg lettuce variety. Previous work with iceberg x romaine combinations were not able to detect the unique genetic concurrence of stature and internal quality of the present invention. [00024] Another common problem that may occur includes gene interactions, whereby the products of the genes interact. Such interaction may prevent the expression of a selected desirable trait. One form of gene interaction is epistasis, whereby interaction between products of non-allelic genes result in modification or masking of the desired phenotype. Epistasis may be brought about by modification of gene function due to alterations in the signal-transducing pathway. Epistasis may also occur indirectly through non-intracellular phenomena. For example, a plant with certain metabolites may exert deleterious effects on a developing embryo which does not normally produce such metabolites.
Another problem in the creation of new varieties includes linkage drag, whereby undesirable genes are brought into a population on the basis of hitchhiking if no recombination occurs between the selected desirable and undesirable genes.
In one aspect of the invention, methods for developing novel plant types are presented. In one preferred embodiment the specific type of breeding method is pedigree selection, where both single plant selection and mass selection practices are employed. Pedigree selection, also known as the xe2x80x9cVilmorin system of selection,xe2x80x9d is described in Fehr, Walter; Principles of Cultivar Development, Volume I, Macmillan Publishing Co., which is hereby incorporated by reference.
In general, selection is first practiced among F2 plants. In the next season, the most desirable F3 lines are first identified, then desirable F3 plants within each line are selected. The following season and in all subsequent generations of inbreeding, the most desirable families are identified first, then desirable lines within the selected families are chosen, and finally desirable plants within selected lines are harvested individually. A family refers to lines that were derived from plants selected from the same progeny row the preceding generation.
Using this pedigree method, two parents may be crossed using an emasculated female and a pollen donor (male) to produce F1 offspring. To optimize crossing, it is important to note that lettuce is an obligate self-pollinating species. This means that the pollen is shed before stigma emergence, assuring 100% self-fertilization. Since each lettuce flower is an aggregate of about 10-20 individual florets, manual removal of the anther tubes containing the pollen is tedious. As such, a method of misting to wash the pollen off prior to fertilization may be employed to assure crossing or hybridization.
Any varieties of iceberg or romaine lettuce may be used as parents in the method of the present invention. More preferably, the parental varieties are selected from commercial varieties that individually exhibit one or more desired phenotypes. Also any breeding method involving selection of plants for the desired phenotype can be used in the method of the present invention.
The F1 may be self-pollinated to produce a segregating F2 generation. Individual plants may then be selected which represent the desired phenotype in each generation (F3, F4, F5, etc.) until the traits are homozygous or fixed within a breeding population.
Although a number of different selection criteria may be used, the selection criteria may include one or more of the following:
1. identification of an iceberg cultivar with the shape and stature of a romaine lettuce
2. a semi-open head
3. dark outer color with blanched inner color.
4. resistance to corky root rot
5. resistance to lettuce mosaic virus
6. length to width ratio of greater than 1.0, and
7. plant height.
The pathogen responsible for corky root is Rhizomas suberifaciens. CA1 is the most common strain and is publicly available from the ATCC (Accession No. 49355). Other useful strains include CA3 and CA15. Colonies of R. suberifaciens are initially translucent but later become opaque. The colonies are umbonate, compact colonies, which ultimately become wrinkled and have raised edges on S-medium as described in Van Bruggen, et al. 1990, Host Range of Rhizomas suberifaciens, the causal agent of corky root of lettuce, Plant Disease, 74:581-584.
R. suberifaciens is an aerobic bacterium, ranging in morphology from small (0.6-1.4.mu by 0.3-0.6.m.u.) rods with one lateral flagellum to long filaments. According to the KOH stringiness test, the bacteria seemed gram-positive, but with Hucker""s gram-stain the bacteria stain gram-negative.
The type CA1 and other equivalent strains are publicly available in the Salinas Valley of California growing in the soil of the lettuce fields. These strains are quite common and can be isolated using the baiting procedure described in Example 5. Characterized strains are also available from Dr. Ariena Van Bruggen at the University of California at Davis.
An initial screen for corky root is initiated using the screening procedure of Example 5. The seeds are sown under greenhouse-controlled conditions in vermiculite soil with a heavy concentration (approx. 107 cfu/ml) or corky root bacteria. Susceptibility is determined by visually inspecting the tap roots for greenish-yellow to gold oblong lesions. Advanced symptoms include a greenish-brown to golden root system and a corky and brown taproot.
The major pathogenic race or pathotype of corky root is CA1, but other strains are known. The cor gene is recognized as a gene responsible for resistance.
Lettuce Mosaic Virus (LMV) is a potyvirus. Natural transmission of LMV is achieved by the green peach aphid (Myzus persicae) feeding on infected host leaves. Wild lettuce and bristly ox-tongue are the two most important natural hosts. The virus is non-persistent in the aphid and seed-borne. The common strain is available from the ATCC. The ATCC accession number is PV-63. Infected leaf tissue can be conveniently stored in a freezer at xe2x88x9220xc2x0 Celsius.
Individual plants are inoculated with LMV as described in Example 6. The virus in ground leaves infected with LMV is mixed with a buffered solution and abraded with carborundum or sand onto the leaf surfaces of the test plants. Inoculation with LMV is at stand establishment when the plants have recovered from transplant shock. Stand establishment is about 3-4 weeks after transplanting, when the plants present two or three expanded leaves. Waiting until stand establishment increases the efficiency of screening for viral-induced mosaic symptoms.
Resistance to LMV is considered under the control of a single recessive gene. Resistant varieties are publicly available as described in Example 6 and can be used as starting material for introducing LMV into the present invention.
The development of all the lines discussed herein (PSR 4569, PSR 6425, PSR 6595, and PSR 6032) used similar selection techniques, although the parental combinations used to make each of the crosses were different. The main factor uniting these combinations of the present invention is the unique phenotype. This invention was facilitated by the selection criteria used in the breeding of these lines where the combination of a romaine shape and stature was linked to other qualities of an iceberg lettuce.
During the breeding and selection of the lines disclosed herein, there was no anticipated phenotype (expected biological design) due to the lack of prior work in this area of lettuce research and breeding. Once an acceptable phenotype had be identified, it became clear that the same desired characteristics could be obtained from a number of different parental combinations. However, this could only be done when the appropriate selection criteria were applied. Thus, only by making repeated selections for the desired phenotype could this invention be repeatedly produced.
The leaf shape of the present invention is spatulate. The term xe2x80x9cspatulatexe2x80x9d as used in this patent includes leaf shapes varying from narrow elliptic or oblong to oblanceolate or broad obtrullate as described in TG/13/7, page 19 supra.
The stature of the present invention is elliptical. The term xe2x80x9cellipticalxe2x80x9d as used in this patent includes statures varying from narrowly elliptical to broad elliptical as described in TG/13/7, page 18 supra.
The length to width ratio of the outer leaf of the present invention is approximately 1.2 to 2.1. However, other embodiments of the invention can comprise ratios varying from about 1.2 to about 2.7. Preferably, the Length to width ratio can vary from 1.25 to 2.2. Most preferably 1.3 to 2.0.
Using Royal Horticultural Society""s Color Chart the outer leaf color of the present invention is normally green (RHS 146B). However, other emodiments of the invention can comprise outer leaf colors varying from dark green (RHS 146A) to green (RHS 146B).
The lettuces of the present invention may have a semi-open head formation. The term xe2x80x9csemi-openxe2x80x9d as used in this patent includes head formations varying from medium to strong as described in TG/13/7, page 13, heading 9 supra.
Although the forgoing invention has been described and illustrated, it should be understood that certain changes and modifications may be practiced within the scope of this invention without departing from the scope of the invention as set forth in the accompanying claims.